I don't think I had one "a-ha!" moment. Whenever we used to get writing and compositions exercises in primary/secondary school I was thrilled. As I grew older, I started realizing that I was one of the very few people in class who actually enjoyed and looked forward to writing exercises.
I found writing to be the best way to traverse through my frantic imagination at the time. Since then I've written many short stories and a few poems (most of them not very good!). It's too bad I don't get to write all that much anymore, though. That said, writing and publishing a novel is still on my bucket-list!